This invention relates to the purification of natural gas, and, more particularly, to the removal of nitrogen from natural gas by use of a molecular sieve in a novel pressure swing adsorption (PSA) process.
The removal of nitrogen from natural gas is of considerable importance inasmuch as nitrogen is present to a significant extent. Nitrogen contamination lowers the heating value of the natural gas and increases the transportation cost based on unit heating value.
Applications aimed at removing nitrogen and other impurities from natural gas streams provide significant benefits to the U.S. economy. In 1993, the Gas Research Insitute (GRI) estimated that 10-15% (xcx9c22 trillion cubic feet) of the natural gas reserves in the U.S. are defined as sub-quality due to contamination with nitrogen, carbon dioxide, sulfur. Most of these reserves, however, have discounted market potential, if they are marketable at all, due to the inability to cost effectively remove the nitrogen. Nitrogen and carbon dioxide are inert gases with no BTU value and must be removed to low levels (4% typically) before the gas can be sold.
Concurrently, the U.S. has proven reserves of natural gas totaling 167 trillion cubic feet. Over the past five years, annual consumption has exceeded the amount of new reserves that were found. This trend could result in higher cost natural gas and possible supply shortages in the future. As the U.S. reserves are produced and depleted, finding new, clean gas reserves involves more costly exploration efforts. This usually involves off shore exploration and/or deeper drilling onshore, both of which are expensive. Moreover, unlike crude oil, it is not economical to bring imports of natural gas into North America, therefore pricing of natural gas could be expected to rise forcing end users to seek alternative fuels, such as oil and coal, that are not as clean burning as gas. While base consumption for natural gas in the U.S. is projected to grow at 2-3% annually for the next ten years, one segment may grow much more rapidly. Natural gas usage in electric power generation is expected to grow rapidly because natural gas is efficient and cleaner burning allowing utilities to reduce emissions. Accordingly, there is a need to develop a cost-effective method of upgrading currently unmarketable sub-quality reserves in the U.S. thereby increasing the proven reserve inventory.
Methods heretofore known for purification of natural gas, in particular, nitrogen removal, may be divided roughly into three classifications:
(a) Methods involving fractional distillation at low temperature and (usually) high pressure, i.e. cryogenics. Since nitrogen has a lower boiling point than methane and the other hydrocarbons present in natural gas, it may be removed as a gas on liquefying the remaining constituents which are then revaporized.
(b) By selective adsorption of the methane and higher hydrocarbons on an adsorbent such as activated charcoal. The adsorbed gases are then desorbed to give a gas free of nitrogen.
(c) Miscellaneous processes involving selective diffusion through a series of organic membranes, formation of lithium nitride by treatment with lithium amalgam, absorption of the nitrogen in liquid ammonia or in liquid sulphur dioxide.
The principal disadvantage of the fractional distillation and adsorption processes is that they remove the major component, methane, from the minor component, nitrogen, instead of the reverse. In cryogenic processing, almost the entire volume of natural gas must be refrigerated, usually compressed, and then heated again. Accordingly, cryogenic processing is expensive to install and operate, limiting its application to a small segment of reserves. Cryogenic technology is believed only capable of cost effectively purifying reserves, which exceed 50,000,000 standard cubic feet of gas per day and as well having nitrogen contamination levels of 15% or more. Gas reserves that do not fit these criteria are not currently being purified. The potential value of this gas is totally lost as the wells are usually capped. The processes suggested under paragraph (c) above are handicapped by an unsatisfactory degree of separation or by the use of very expensive materials.
In smaller-scale natural gas operations as well as in other areas such as synthesis gas and coke oven gas processing, adsorption processes can be especially well suited. The adsorption capacities of adsorption units can, in many cases, be readily adapted to process gas mixtures of varying nitrogen content without equipment modifications, i.e. by adjusting adsorption cycle times. Moreover, adsorption units can be conveniently skid-mounted, thus providing easy mobility between gas processing locations. Further, adsorption processes are often desirable because more than one component can be removed from the gas. As noted above, nitrogen-containing gases often contain other gases that contain molecules having smaller molecular dimensions than nitrogen, e.g., for natural gas, carbon dioxide, oxygen and water, and for coke oven gas, carbon monoxide.
U.S. Pat. No. 2,843,219 discloses a process for removing nitrogen from natural gas utilizing zeolites broadly and contains specific examples for the use of zeolite 4A. The process disclosed in the patent suggests use of a first nitrogen selective adsorbent zeolite in combination with a second methane selective adsorbent. The molecular sieve adsorbent for removing nitrogen is primarily regenerated during desorption by thermal swing. A moving bed adsorption/desorption process is necessary for providing sufficient heat for the thermal swing desorption. The moving bed process specifically disclosed in this patent is not practical and it does not provide a cost efficient method for the separation of nitrogen from natural gas in view of high equipment and maintenance costs and degradation of the adsorbent by attrition due to contact with the moving adsorbent particles.
Despite the advantageous aspects of adsorption processes, the adsorptive separation of nitrogen from methane has been found to be particularly difficult. The primary problem is in finding an adsorbent that has sufficient selectivity for nitrogen over methane in order to provide a commercially viable process. In general, selectivity is related to polarizability, and methane is more polarizable than nitrogen. Therefore, the equilibrium adsorption selectivity of essentially all known adsorbents such as large pore zeolites, carbon, silica gel, alumina, etc. all favor methane over nitrogen. However, since nitrogen is a smaller molecule than methane, it is possible to have a small pore zeolite which adsorbs nitrogen faster than methane. Clinoptilolite is one of the zeolites which has been disclosed in literature as a rate selective adsorbent for the separation of nitrogen and methane.
U.S. Pat. No. 4,964,889 discloses the use of natural zeolites such as clinoptilolites having a magnesium cation content of at least 5 equivalent percent of the ion-exchangeable cations in the clinoptilolite molecular sieve for the removal of nitrogen from natural gas. The patent discloses that the separation can be performed by any known adsorption cycle such as pressure swing, thermal swing, displacement purge or nonadsorbable purge, although pressure swing adsorption is preferred. However, this patent is silent as to specifics of the process, such as steps for treating the waste gas, nor is there disclosure of a high overall system recovery.
In general, first applications of PSA processes were performed to achieve the objective of removing smaller quantities of adsorbable components from essentially non-adsorbable gases. Examples of such processes are the removal of water from air, also called heatless drying, or the removal of smaller quantities of impurities from hydrogen. Later this technology was extended to bulk separations such as the recovery of pure hydrogen from a stream containing 30 to 90 mole percent of hydrogen and other readily adsorbable components like carbon monoxide or dioxide, or, for example, the recovery of oxygen from air by selectively adsorbing nitrogen onto molecular sieves.
The carrying out of the PSA processes in multi-bed systems is illustrated by the Wagner patent, U.S. Pat. No. 3,430,418, relating to a system having at least four beds. As is generally known and described in this patent, the PSA process is commonly performed in a cycle of a processing sequence that includes in each bed: (1) higher pressure adsorption with release of product effluent from the product end of the bed; (2) co-current depressurization to intermediate pressure with release of void space gas from the product end thereof; (3) countercurrent depressurization to a lower pressure; (4) purge; and (5) pressurization. The void space gas released during the co-current depressurization step is commonly employed for pressure equilization purposes and to provide purge gas to a bed at its lower desorption pressure.
Similar systems are known which utilize three beds for separations. See, for example, U.S. Pat. No. 3,738,087 to McCombs. The faster the beds perform steps 1 to 5 to complete a cycle, the smaller the beds can be when used to handle a given hourly feed gas flow. If two steps are performed simultaneously, the number of beds can be reduced or the speed of cycling increased; thus, reduced costs are obtainable.
U.S. Pat. No. 4,589,888 to Hiscock, et. al. discloses that reduced cycle times are achieved by an advantageous combination of specific simultaneous processing steps. The gas released upon co-current depressurization from higher adsorption pressure is employed simultaneously for pressure equalization and purge purposes. Co-current depressurization is also performed at an intermediate pressure level, while countercurrent depressurization is simultaneously performed at the opposite end of the bed being depressurized.
U.S. Pat. No. 4,512,780 to Fuderer discloses a pressure swing adsorption process with intermediate product recovery. Three products are recovered from a pressure swing adsorption process utilizing a displacement step in conjunction with pressure equalization between beds of a multi-bed adsorption system. This process is not cost efficient for the recovery of two products.
PSA processes were first used for gas separations in which only one of the key components was recovered at high purity. For example, from 100 mols feed gas containing 80 mols hydrogen and 20 mols carbon monoxide, the process of Wagner, U.S. Pat. No. 3,430,418, or of Hiscock, et. al., U.S. Pat. No. 4,589,888, could separate 60 mols of hydrogen at 99.999% purity, but no pure carbon monoxide could be recovered; 20 mols of carbon monoxide and 20 mols of hydrogen remained mixed at 50% purity each. Neither of these processes can make a complete separation. Only the less adsorbable, light component is recovered at high purity.
For the recovery of a pure, stronger adsorbed, xe2x80x9cheavyxe2x80x9d component, an additional step is necessary, namely, rinsing of the bed with a heavy component to displace the light component from the bed prior to depressurization. The rinsing step is described in several earlier patents. The problems with these processes are the following: (a) if the rinsing is complete and the light component is completely displaced from the bed, pure heavy component can be obtained, but the adsorption front of the heavy component breaks through to the light component and the latter cannot be recovered at high purity; (b) if the displacement of the light component is incomplete, the typical concentration profile of the heavy component in the bed is not optimum and such bed is depressurized countercurrently to recover the heavy key component at the feed end, the light component still present in the bed reaches the feed end very rapidly and the purity of the heavy component drops. Therefore, it is not practical with the prior art processes to obtain both key components at high purity in a single PSA unit.
Such complete separations can be obtained, for example, by two separate pressure swing adsorption processing units wherein each unit includes several fixed beds. From a feed gas containing, for example, hydrogen and carbon monoxide (CO), the first unit recovers pure hydrogen and a carbon monoxide rich gas containing 70% carbon monoxide. This gas mixture is compressed and passed through a second PSA unit which recovers pure carbon monoxide and a hydrogen rich gas. The hydrogen rich gas can be added as feed gas to the first PSA unit and then the cycle is repeated. The combination of two independent PSA units can make an excellent separation at very high flexibility. For example, from a gas mixture with two components this system can recover more than 99.8% of the adsorbable xe2x80x9clightxe2x80x9d component such as hydrogen at a purity of 99.999% and also recover essentially 100% of the more readily adsorbed component such as carbon monoxide at a purity higher than 99.5%.
Although pressure swing separation adsorption (PSA) has been used to separate a wide variety of gases, the simple fact remains that there is no commercially practiced PSA process for the separation of nitrogen from methane. This is due to many factors including the lack of a nitrogen specific adsorbent and environmental regulations.
As previously pointed out, a significant percentage of U.S. natural gas reserves contain more than 4% nitrogen. The bulk of these reserves cannot be exploited because no economical technology exists for removing nitrogen especially at low flow rates, i.e., less than 25 MMSCFD process feed gas. Cryogenic distillation is the only process being used to date on any scale to remove nitrogen from methane in natural gas. Cryogenic plants are not used more widely because they are expensive and complicated and exhibit poor scale down economics.
It is the primary objective of this invention to provide a commercially viable PSA process for removing nitrogen from natural gas.
A further object of the invention is to provide a PSA process for removing nitrogen from natural gas which can provide a highly concentrated methane product at high process efficiencies.
Another object of this invention is to separate nitrogen from natural gas by a novel PSA process which yields a methane product, and a high purity nitrogen stream.
Still another object of the invention is to provide and maintain peak efficiency of the nitrogen-selective adsorbent during PSA separation of nitrogen from natural gas.
This invention provides a PSA system to achieve separation of nitrogen from natural gas with high system recovery of separate methane and nitrogen product streams from the feed gas. This is accomplished by placing a methane selective adsorbent in a PSA system on the waste stream of a nitrogen-selective PSA system in order to boost the methane recovery of the system and further generate a high purity nitrogen gas stream that could be utilized for other related unit operations or vented to the atmosphere.
In another aspect of the invention, the methane-enriched waste stream from the methane selective PSA is cooled to remove C3+ hydrocarbons before being recycled to feed so as to eliminate the build up of heavy hydrocarbons in the PSA units.